1. Field of the Invention
The present invention relates to the preparation of noble metal catalysts having high sulphur tolerance for the hydrogenation of aromatic compounds present in middle distillates. In particular, the invention concerns a catalyst with high sulphur tolerance and comprising a noble metal on a surface-modified alumina or silica support. The new catalysts can be used for hydrogenation of middle distillate aromatics to obtain diesel fuel.
2. Description of Related Art
Nickel-based catalysts have been conventionally used for the hydrogenation of aromatic compounds. The performance of these catalysts in a sulphur-containing feed deteriorates with time owing to poisoning of nickel. The activity of the catalysts is not restored when changing a sulphur-containing feed to a pure, i.e. no sulphur compounds containing, feed (irreversible process). Regeneration of nickel catalysts with hydrogen is not successful either. Owing to very low sulphur tolerance and inability to be regenerated nickel-based catalysts are in general not used with sulphur-containing feeds.
An alumina-based noble metal catalyst, for example platinum on gamma-alumina, is known to have high catalytic activity in hydrogenation of aromatic compounds. However, it is also very sensitive to sulphur, and therefore it is necessary to decrease the amount of sulphur in the feed to a very low level (&lt;5 ppm). Contrary to nickel, the activity of a platinum catalyst is restored when changing sulphur-containing feed to a pure, i.e. no sulphur compounds containing, feed (reversible process).
It is already known that the amount of sulphur in the feed can be increased if the acidity of the support of a noble metal catalyst is increased, for example, by changing an alumina support to the amorphous silica-alumina support. Below, there is a brief description of the patents in which the sulphur tolerance of platinum-based hydrogenation catalysts has been improved by adding platinum onto a silica-alumina mixed oxide (U.S. Pat. No. 3,269,939), onto a supported silica-alumina (U.S. Pat. No. 3,703,461) and onto a dealuminized silica-alumina (GB 1,501,346).
U.S. Pat. No. 3,269,939 teaches a silica-alumina support for a platinum based (0.75 wt. % Pt) catalyst, having a silica content in the range of 75 to 90 wt. % (SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio=86/14). According to the reference, the use of silica-alumina as the support renders the catalyst more tolerant to sulphur as compared with alumina alone or silica-alumina in which the SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio is reversed (12/88). The enhancement of hydrogenation which breaks down sulphur compounds on the support concerned is assumed to be the reason for the better sulphur tolerance of the catalyst. The sulphur tolerance of the catalyst at different sulphur concentrations is presented in an example in which a feed containing 17 vol. % aromatics was hydrogenated (T=300.degree. C., p=35 bar, space velocity=6 hr.sup.-1, hydrogen to oil ratio=500 N1/1). When the sulphur contents in the feed were 50 ppm and 300 ppm, the conversions of the aromatics were 62% and 44%, respectively.
U.S. Pat. No. 3,703,461 describes a support comprising a large-pore alumina gel into which a silica-alumina cogel or copolymer is dispersed. The noble metals are added to the support by the ion exchange method in order to achieve good dispersion. When using suitable silica-alumina mixtures, a sufficient number of ion exchange sites are obtained for the noble metals, and the large pores ensure that the well dispersed noble metals are also available. According to the patent, the noble metals are in effective use, owing to synergy between the large-pore support and the ion exchange method. Furthermore, the improved sulphur tolerance of the catalyst concerned (0.6 wt. % Pt) as compared with a commercial reformation catalyst (0.55 wt. % Pt/Al.sub.2 O.sub.3) is shown by means of an example. When the sulphur content in the feed was 73 ppm during a 700-hour run, the hydrogenation of aromatics (19 vol. %) remained complete (T=315.degree. C., p=75 bar, space velocity=2.3 hr.sup.-1, hydrogen to oil ratio=980 N1/1). In a corresponding run the conversion level of the reference catalyst decreased steadily. GB 1,501,346 discloses a sulphur-tolerant noble metal catalyst, the support of which is dealuminized silica-alumina. It is stated that the high sulphur tolerance of the catalyst is based on the optimal surface structure produced in the dealumination of the support, the acidity of the surface being here of crucial importance. In the examples, the sulphur tolerance of the platinum catalysts (0.1-0.8 wt. % Pt) was tested by using the feeds in which sulphur content ranged from 80 to 500 ppm. When the sulphur content in the feed was 500 ppm, a catalyst containing 0.6 wt. % platinum hydrogenated (T=320.degree. C., p=50 bar, space velocity=4.0 hr.sup.-1, hydrogen to oil ratio=1000 N1/1) 90% of the aromatics (initially 22.8 wt. %).
It is known that the acidity of the alumina support surface can also be increased by using, for example, the oxides of boron (B.sub.2 O.sub.3), phosphorus (PO.sub.x) niobium (Nb.sub.2 O.sub.5), titanium (TiO.sub.2), tungsten (WO.sub.3) and zirconium (ZrO.sub.2). The same oxides are also suitable for increasing the acidity of the silica support, as is the adding of, for example, gallium oxide (Ga.sub.2 O.sub.3) and lanthanum oxide (La.sub.2 O.sub.3). The above-mentioned oxides mixed together also form oxide structures which have an acidic nature.
For the hydrogenation of middle distillate aromatics there have been developed zeolite-based (crystalline silica-alumina) noble metal catalysts (Pd, Pt and a mixture thereof) which have a higher tolerance of sulphur and nitrogen than have noble metal catalysts based on alumina and silica-alumina. The support is most commonly Y-type zeolite or mordenite. It has been suggested in the art that the sulphur tolerance of zeolite-based catalysts would be based on an electron deficiency of the noble metal particles, caused by the zeolite structure, in which case the strength of the bond between the noble metal and sulphur is weakened (A. Stanislaus and B. H. Cooper, Catal. Rev.--Sci. Eng. 36 (1994) 75). For the hydrogenation of aromatic compounds, it is also possible to use molecular sieves, for example MCM-41Al.sub.2 O.sub.3, in which noble metals of Group VIII of the Periodic Table act as the catalytically active metals.
U.S. Pat. No. 5,114,562, WO92/16601 and WO96/09360 describe two-step processes wherein aromatics are hydrogenated with noble metal catalysts after the removal of sulphur. The supports proposed include both zeolite type and non-zeolite type (such as alumina and amorphous silica-alumina) oxide materials. However, these publications do not discuss the differences caused by different supports in the sulphur tolerance of the noble metal catalysts.
In order to complete the survey of related art, it should be mentioned that a method for improving thermal and mechanical stability of alumina by treating it with a silicon compound is disclosed in U.S. Pat. No. 4,013,590. According to said reference the aluminum oxide is impregnated with the silicon compound or a solution thereof, the impregnated aluminum oxide is dried and then subjected to oxidation. There is no indication of the suitability of the thus treated alumina for use as a support for nobel metal catalysts employed for hydrogenation of sulphurous feeds.